1. A Comparative Evaluation of Cortical Thickness Measurement Techniques P.A. Bromiley, M.L.J. Scott, and N.A. ThackerImaging Science and Biomedical EngineeringUniversity of Manchester

2. Introduction • The cerebral cortex: • largest part of the brain • highly convoluted 2D sheet of neuronal tissue • laminar structure • min. thickness ~2mm (calcarine sulcus) • max. thickness ~4mm (precentral gyrus) • av. thickness ~3mm

3. Introduction • Volume measurements are well established • e.g. dementia, ageing • Thickness provides additional information • correlations with Alzheimer’s, Williams syndrome, schizophrenia, fetal alcohol syndrome…

4. Introduction • Free from region definition v t

5. v1 v2 Introduction • More robust to misregistration • volume error  misregistration

6. Introduction • More robust to misregistration • median thickness error  t / n

7. Introduction • Two approaches: • model based (e.g. ASP, McDonald et al. 2000) • fit deformable model to inner surface • expand to reach outer surface • measure distance between corresponding vertices • data-driven • use edge detection to find inner surface • find 3D normal • search along normal for another edge

8. The problem… • Partial volume effect may obscure outer surface (from McDonald et al. 2000)

9. Model Bias • Impose constraints the force spherical topology and force the models into thin sluci: • distance between vertices on inner and outer surfaces • surface self proximity • may introduce bias • takes ages to run

10. The TINA Cortical Thickness Algorithm • Scott et al., MIUA 2005 • find inner surface • search along 3D normal • process edges, dips found

11. AIM • Can data driven techniques be as accurate as model-based ones? • Can we find evidence of model bias?

12. Evaluation • 119 normal subjects, 52 male, age 19-86 (μ=70.3) • T1-weighted IR scans: suppresses inhomogeneity

13. Evaluation • Meta-studies: • youngest 13 compared to Kabani et al. manual and automatic (model based) • precentral gyrus thickness vs. age compared to 8 previous publications for all 119 subjects …if we can see aging, we can see disease

14. Comparison to Kabani et al.

15. Comparison to Kabani et al.

16. Comparison to Kabani et al. • From error propagation, expected error on an individual ~0.1mm • Mean differences • present study: –0.21 +/- 0.22 mm • Kabani et al.: 0.61 +/- 0.43 mm • => mostly group variability • No evidence of systematic error • Data-driven technique has ~2x lower random errors

17. Precentral Gyrus Study • Meta-study incorporating 635 subjects: Reference No. Age range (years) Algorithm type Kabani et al. (2001) 40 18-40 Model based Von Economo (1929) - 30-40 Manual measurement Sowell et al. (2004) 45 5-11 Intensity based Tosun et al. (2004) 105 59-84 Model based Fischl et al. (2005) 30 20-37 Model based Thompson et al. (2005) 40 18-48 Intensity based MacDonald et al. (2000) 150 18-40 Model based Salat et al. (2004) 106 18-93 Model based Present study 119 19-86 Intensity based

18. Precentral Gyrus Study • Colourmap representations • error estimation is not possible • bias from inflated/non-inflated representations • (from Fischl et. al., 2000)

19. Precentral Gryus Study

20. Conclusions • Results from all other studies are consistent • random errors dominated by natural variation • Data-driven cortical thickness measurement • free from model bias • order of magnitude faster • at least as accurate …compared to model-based techniques • Bias may have been seen in the Salat et al. results? • don’t use prior measurement to make measurement